1. Thin Film Physics Division, Department of Physics (IFM), Linköping University, SE-581 83 Linköping, Sweden
2. SweGaN AB, Teknikringen 8D, SE-583 30 Linköping, Sweden
3. Institute of Electronic, Microelectronic and Nanotechnology, Av. Poincaré, 59650 Villeneuve d'Ascq, France